This invention relates to a method of characterizing the occupant of a motor vehicle for purposes of enabling or disabling air bag deployment based on sensed occupant weight, and more particularly to a method of distinguishing a cinched child seat from a small adult or child of similar apparent weight.
Vehicle occupant weight detection systems are useful in connection with air bags and other pyrotechnically deployed restraints as a means of characterizing the occupant for purposes of determining whether to enable or disable deployment of the restraints. For example, it is generally desired to enable deployment for a child or small adult, and to disable deployment (or reduce deployment force) for a small child. In the case of infant or child seats (referred to herein collectively as child seats) that are placed on the vehicle seat and cinched down with a seat belt, it is generally believed that deployment should be disabled entirely. Unfortunately, there can be some ambiguity in the case of a child seat, particularly if the seat belt restraining the child seat is cinched very tightly, as a tightly cinched child seat can produce a weight reading similar to that of a small adult (5th percentile female). Accordingly, various attempts have been made to distinguish a child seat from other occupants producing a similar weight reading. In certain systems, for example, a special tag is affixed to the child seat for detection by a sensor located in the seat back or instrument panel. It has also been proposed to measure the seat belt restraining force to determine if a significant portion of the sensed weight is due to a cinched seat belt. However, both of these approaches are difficult to implement in an inexpensive and reliable manner.
A cost-effective and generally reliable method of distinguishing between a small adult and a tightly cinched child seat is disclosed in co-pending and co-assigned U.S. patent application Ser. No. 09/412,936, filed on Oct. 5, 1999. According to the disclosed method, an occupant producing a weight reading in a range that includes tightly cinched child seats is distinguished from a child seat by detecting the variation in the sensed weight during vehicle movement. The occupant is characterized as a child or small adult if the variation exceeds a threshold, whereas the occupant is characterized as a child seat if the variation is below the threshold, as a tightly cinched seat belt severely restricts variance. In fact, vehicle movement itself can be inferred from the weight variance, if desired. The method also detects a frequency of the sensed weight as a correlative factor for distinguishing between a child seat and a child or small adult. However, testing has revealed that this method cannot reliably identify the presence of a cinched child seat when the vehicle is being operated on a rough road surface. This is because the rough road increases the range of weight variance for a cinched child seat to the point that it partially overlaps the normally expected range of weight variance for a child or small adult. Accordingly, what is desired is an occupant characterization method that can reliably identify the presence of a cinched child seat, even when the vehicle is being operated on a rough road.
The present invention is directed to an improved weight-based occupant characterization method that reliably distinguishes between a child or small adult and a tightly cinched child seat, based on a variance in the sensed occupant weight and a variance in the vehicle acceleration. According to the invention, the weight variance and acceleration variance are used to determine a relative or normalized variance that compensates for the effects of operating the vehicle on a rough road surface. The occupant is characterized as a cinched child seat if the normalized variance is below a first threshold for a predetermined interval, and as a normally restrained child or small adult if the normalized variation exceeds a second threshold for a predetermined interval.